The present invention relates generally to the art of wrapping loads with a stretchable plastic sheet of film, and more specifically to a roller structure especially adapted for precise, uniform transportation and stretching of plastic sheet of film during the wrapping process between the dispenser roll and the load.
Case packing or boxing is a common way of shipping multiple unit products. The multiple unit products are generally stacked in a corrugated box or are wrapped with kraft paper with the ends of the kraft paper being glued or taped. Another way of shipping such products is by putting a sleeve or covering of heat shrinkable film around the products and shrinking the sleeve to form a unitized package. The use of heat shrinkable film is described in U.S. Pat. No(s). 3,793,798; 3,626,645; 3,590,509; and 3,514,920. A discussion of this art is set forth in U.S. Pat. No. 3,867,806.
Another common method of wrapping load is with rotary stretch wrapping machines. These rotary machines are commonly referred to as spiral or full-web machines, and can operate with the load rotating to pull stretched film web around it. Alternatively, the load can be stationary and stretched film wrapped around the load with a rotating film dispenser.
A typical film-web apparatus is disclosed in U.S. Pat. No. 3,867,806.
The use of spiral wrapping machinery is well known in the art and representative machines are typified by U.S. Pat. No(s). 3,003,297; 3,788,199; 3,683,425; and 4,136,501.
Additional references of interest which are pertinent to rotatable drives for wrapping packages are disclosed in U.S. Pat. No(s). 3,820,451; 3,331,312; 3,324,789; 3,309,839; 3,207,060; 2,743,562; 2,630,751; 2,330,629; 2,054,603 and 2,124,770.
The film stretching means on many currently marketed pallet stretch wrapping devices employ either direct or indirect friction to restrict the film as it is being wound onto the load during the wrapping process. The restriction is either applied to the roll of film itself (direct friction) or applied to the film after it is unwound from the film roll (indirect friction). The pallet and load serve as the winding mandrel providing all of the pulling force required to elongate the film.
The earliest type of stretch wrapper utilizes a direct friction device in the form of a brake that is connected to the core of the film roll. The torque from the friction brake device acts on the center of the film roll and as the diameter of the roll is reduced, the voltage to the brake is altered, either by the operator or automatically by a sensing device. A later film roll brake device, illustrated by U.S. Pat. No. 4,077,179, and FIG. 2 herein, utilizes a frictional brake attached to a shaft with a roller which is pressed against the freely mounted film roll. The film roll brake eliminates the need to change the brake force during the consumption of the film roll.
Various prior art indirect friction film stretching devices have been employed to restrict the film as it is wound onto the pallet during the wrapping process. One of these devices, commonly referred to as an "S" type roller device, utilizes an idle roller followed by a braked roller over which the film is threaded prior to wrapping the load. The function of the idle roller is to align the film for maximum contact with the braked roller. Another indirect friction device having fixed bars was marketed by Radient Engineering Corporation under the trade name POS-A-TENSIONER and has been subsequently marketed by the Kaufman Company under the trade name TNT. This device has a series of fixed, non-rotating bars positioned adjacent to the film roll. The film web is threaded around the bars whose relative angles can be changed for ultimate tensioning. As the film web is drawn to the pallet it passes across the bars, and the friction between the film and the smooth surface of the bars provides a restriction causing the film to stretch. This device uses multiple bars with the film web stretching incrementally between each bar. Neck down of the film web increases between each bar and the load bears the force. As the load rotates, the wrap angle changes from the last bar so that the wrapping force greatly varies depending on the relative angles. The frictional restraint is determined by the vector of the film web on each bar. Thus, the device is very sensitive to the force placed on the supply roll, and the force increases as the roll size decreases, adding additional force on the system. Furthermore, there must be some friction placed on the supply roll to prevent backlash. While this device solves to some degree the irregularities of the brake and hostility of the film roll, it can only apply limited stretch to the load and does not handle different film compositions with any degree of standardization.
Another stretch wrapper device was introduced by the Anderson Company at the PMMI Show in Chicago in 1978. This device interconnects the turntable drive motor with a pair of nip rollers immediately downstream from the film supply roll. The nip rollers are synchronously driven with the turntable rotation through a variable transmission which could be increased or decreased in speed relative to the turntable rotation speed. Thus the stretch on the film was effected between the nip rollers and the pallet load. It is not known if this machine was ever commercialized, principally because of its inability to achieve satisfactory stretch over the load corners due to its failure to respond to the speed change that these corners represented. The pallet load, as the film accumulating mandrel, provided the total force that was required to stretch the film from the driven nip rollers with all of the stretch occurring after the passage of the single pair of nip rollers to the pallet.
In addition to the previously noted prior art, direct friction pallet stretch wrapping machines of the pass through type have been manufactured by Weldotron and Arenco (Model No. MIPAC). These machines have a significant problem in stretching the film and normally stretch film around the load in an elongation range of about five to ten percent. These machines depend on being able to drive the pallet and associated load through a stretch curtain of film to place the stretching force on the front or sides of the load. Since most pallet loads will not hold together while being subjected to these unequal forces, the film web is normally tensioned after the film seal jaws begin their inward travel over the end of the pallet load. This form of tensioning severely reduces the maximum degree of film elongation and pulls excess film around the two rear corners of the load while the jaws are closing. This frequently causes film tears when the film is stretched more than ten percent.
When low stretch rates of one to ten percent are produced, several packaging problems occur. The unitizing containment forces on the load are less than the optimum force which can be obtained. The minimal containment forces can result in a potential loosening of the film wrap during shipment when the load settles and moves together thereby reducing the girth.
French Pat. No. 2,281,275 assigned to SAT discloses the pre-stretching of plastic film by taking the film web from the film roll through a powered roller system having a speed differential of V.sub.2 -V.sub.1 which stretches the film. The film leaving the second set of rollers is drawn off at a speed which is equal to or less than V.sub.2, the speed of the stretched film coming off of the second roller assembly.
The French Patent achieves film web stretch with various problems. The system requires manual operation or complex automatic feedback to accommodate the changes in film take-up speed as the pallet load surfaces pass by the downstream rollers. This reference does not teach the benefit of stretching the film above the yield point with increased strength per cross-sectional area and increase in modulus. There is furthermore no teaching of reducing the force on the portion of the film web between the downstream powered rollers and the load with inelastic strain recovery as a technique for reducing wrapping force while holding high levels of elongation.
A commercial model based on FIG. 8 of the '275 reference has been marketed by SAT. In this embodiment the film web is pre-stretched by extending a pair of rollers forward while braking the film rolls. The load is carried into the pre-stretched "U" shaped sleeve and the rollers are transported behind the load allowing the sleeve to engage the load. Sealer bars are then projected inward to seal the web ends together.
The aforementioned stretching devices do not maintain a consistent force in stretching the film web. These brake devices are subject to variation due to their physical construction and their sensitivity to speed change caused by passage of corners of the load and the resultant sudden speed-up and slow-down of film drawn from the feed roll.
The elasticity of the stretched plastic film holds the products of the load under more tension than either the shrink wrap or the kraft wrap, particularly with products which settle when packaged. The effectiveness of stretch plastic film in holding a load together is a function of the containment or stretch force being placed on the load and the ultimate strength of the total layered film wrap. These two functions are determined by the modulus or hardness of the film after stretch has taken place and the ultimate strength of the film after application. Containment force is currently achieved by maximizing elongation until just below a critical point where breaking of the film occurs. Virtually all stretch film on the market today including products of Mobil Chemical Company (Mobil-X, Mobil-C and Mobil-H), Borden Resinite Devision PS-26, Consolidated Thermoplastic, Presto, PPD and others are consistently stretched less than the manufacturer's laboratory rated capacity which frequently is in excess of three hundred percent.
This problem of obtaining less stretch on commercial wrapping than that available under laboratory conditions centers on several facts. A square or rectangular pallet which is typically positioned off of its center of rotation is used as the wind up mandrel for the purpose of stretching film. A typical 40".times.48" pallet positioned 3 to 4 inches off of its center of rotation will experience a speed change of up to 60% within one quarter revolution of the turntable.
In addition to the off centering problem, most pallet loads are irregular in shape with vertical profiles which produce a significant puncture hazard to highly stretched film being wound around them. Further, some unit loads are very susceptible to crushing forces of the stretched film. Because of pallet load changes and inconsistencies within the film roll, the operator typically continues to reduce the tension settings until there are no failures. Thus the inconsistencies of films, stretching devices, and pallet loads produce an environment where very few stretch films are actually stretched to their optimum yield.
The major problem with prior stretch technology is that stretch is produced by frictional force devices to restrict the film travel between two relatively hostile bodies. On the one hand the film roll is subject to edge wandering and feathering, while on the other hand the rotating pallet with its irregular edges and rapidly changing wind-up speeds severely limits the level of elongation achieved. The ultimate holding forces of the film cannot be brought to bear on the load because the film cannot be stretched enough. Even if the film could be stretched enough the high wrapping forces can disrupt or crush many unit loads. The use of high modulus films, such as oriented films, does not produce the yield benefits of the current invention, since these higher modulus films would have to be significantly stretched in order to achieve the rubber band effect and moldability required for irregular loads.
It therefore can be understood, since the pallet provides the forces for stretching the film, that stretch percentages achieved on the pallet and the stretch force achieved are intertwined in all prior art devices. As previously indicated, high stretch percentages are required to achieve the benefits of high yield but the high stretch forces necessary for these high stretch percentages cause premature film rupture and potential crushing of the load.
In an attempt to solve the aforementioned problems several other devices have been developed.
One film stretching device called the powered stretch embodiment stretches the film web above its yield point between two sets of powered rollers prior to transporting the film to the pallet, increasing its modulus while reducing its cross-sectional area.
Since the film stretches between the rollers, all stretching action is isolated from the roll and the pallet. It also moves the dependence of the stretch force and elongation level. While the device can be used to wrap light or crushable loads it has several problems in actual use. The controls necessary to compensate for the interacting speed changes are very complex and prohibitively expensive. Thus, the device generally will require feedback controls to sense force change and maintain the force level.
Another known device manufactured by Lantech Inc., under the trademark "ROLLER STRETCH" utilizes the film web to drive the apparatus. This device addresses several of the aforementioned problems. Since the film is pre-stretched between the rollers, it isolates the stretching action from both the film roll and pallet load. This device provides a consistent level of stretch and, most importantly, responds to force and speed changes without complex feedback controls. A problem inherent with the ROLLER STRETCH device is that it has a dependence between the percentage of stretch that can be achieved and the stretch force for a given elongation level. This is due to the mechanical advantage between the film driven rollers.
A further development is disclosed in U.S. Pat. No. 4,387,552 assigned to Lantech, Inc. In this apparatus film web is drawn from a supply roll and across the surfaces of two rollers by rotation of the load to be wrapped to which the leading edge of the film web is attached. The rollers are geared for proportional rates of rotation, and their speeds are varied by the varying take up of film web at the non-symmetrical load surface. A torque is contributed to the downstream roller so that the mutual force exerted on the load and the film web at the load is reduced, thereby minimizing the risk of film web rupture and of load collapse. The ratio of the gears between the rollers is selected so that the film web is stretched over its yield point, which provides a substantial film material costs savings as well as improved holding strength on the load.
The rollers utilized in the film web path between the supply roller and the load have commonly been adapted from those used in the conveyor industry as lagging head pulleys to drive endless conveyor belts. These rollers are coated with neoprene, urethane, or solid plastisol. However, experimentation and commercial usage have revealed that these roller structures do not provide durability and performance consistency, and so impede the desired use of the wrapping systems to unitize loads at maximum throughput while avoiding film rupture and load collapse.
Neoprene-coated rollers have been prepared using a vulcanizing process. Neoprene is typically obtained in sheet form approximately of the thickness desired in the ultimate coating around the roller. Such sheets are wrapped around the roller core, baked until adjacent neoprene portions and edges melt and merge, and then allowed to cool. The resulting surface is irregular in cross section, and must be machined to obtain a cylindrical surface centered on the axis of the underlying core. The surface exposed by machining is a smooth non-porous surface.
With neoprene-coated rollers, film under tension puts cuts and grooves in the plastic surface of the roller. Various portions of the roller surface can be damaged when the supply roll is exchanged for a shorter or longer one corresponding to wider or narrower film web, because the edges of the film cause a significant portion of the damage. Also, the film web supply roll and pre-stretch rollers are moved along their vertical axes in spiral pallet wrapping systems while the film web is dispensed to create a spiral wrap pattern about the load. This complex motion introduces additional forces on the web at the rollers which contributes to a more complex wear pattern. Ultimately, the neoprene is worn away to expose the metallic roller core. Variations in the wear pattern on the rollers introduce corresponding variations in effective roller diameter and, therefore, variations in lengthwise film stretch. Thus the film web may be stretched at a higher or lower percentage between adjacent bands or strips along the length of the web, and certain film web portions become more prone to rupture than others. The lack of uniformity impedes operation of the wrapping procedure at maximum efficiency.
Urethane rollers have been prepared using a mold process. Urethane is commonly delivered as a binary formulation comprising a base and a blowing agent which are mixed immediately prior to use. The mixture process is considered critical for proper end results: together with the details of the formulation, the mixture can control an extremely broad range of characteristics of the final plastic product. Mixing is normally conducted in an automated device, although small amounts can be prepared by hand and such manual activities is considered a skilled art form conducted by experts. After mixture is completed, the substance is heated and poured into a heated mold surrounding the roller core. The mold and core are maintained in place until cool and the mold is then separated so that the coated core may be removed.
The deficiency of urethane-coated rollers relates to the chemical formulation of the film web itself. Many popular webs for wrapping include an additive which promotes the ability of the web to cling to itself. This tackiness additive provides the commercial advantage of being able to seal a completed overwrap merely by wiping a severed trailing end against an underlying layer of the same material. In many situations this is considered a reliable and economical manner of sealing a package. However, tackiness additives collect on urethane-coated pre-stretch rollers in a random and non-uniform pattern. As a result, film moving across the rollers during wrapping does not depart from the roller surface at a tangent as would be expected in an ideal system. Rather, each surface point of any given film web cross-section deposits a portion of its own tackiness additive to the roller and adheres to the roller for some distance beyond the tangent point. Moreover, that distance may not be the same as the corresponding distance for adjacent portions of the cross-section. The film web therefore experiences varying radial forces relative to the axis of the roller as well as varying tangential forces which stretch the film web. As a further result, a large proportion of film web failures experienced during pre-stretch wrapping arise between the two pre-stretch rollers rather than upstream from the first or downstream from the second. Each such failure requires the machine operator to cease the wrapping procedure and re-thread the film web. Therefore, the failure also contributes to economic inefficiency.
In response to the difficulties evident in the prior art, another type of roller coating was developed. This coating was a plastisol obtained from MR Plastics and Coatings, and is known as Mystaflex 428-V. This substance provides a coating with a hardness rating in the range of 70 to 80 durometer. Therefore it exhibits improved resistance to wear over that which had been experienced with neoprene. Plastisol is delivered as a liquid at room temperature, and the roller core is heated and then dipped into the liquid. The length of time during which the roller is submerged in the liquid plastisol determines the thickness of the coating which results. The coating surface is initially neither perfectly cylindrical nor centered on the axis of the underlying roller. Therefore, the surface is machined to achieve the desired coaxial cylinder surface. In this prior art coating, the surface after machining was smooth and non-porous, reflecting the fact that this and most plastisols are uniformly solid throughout. Further, the roller was machined to produce circumferential, circular grooves which increased friction by allowing web to partially collapse into the grooves. However, these grooves do not contact film web so no tackiness additive can build up therein. This reduced the tendency of the film web to stick to the roller past the point of tangent separation, but only for film over the grooves. Film still slipped around the roller circumference and ruptured frequently. Ultimately, use of such a solid plastisol coated roller was shown to be acceptable for only a narrow class of film formulations and wrapping operation modes. Variation of the width and spacing of the grooves on the rollers was required in order to accommodate different film types or differing wrapping machines. This clearly results in inefficiency of machine manufacture which is passed on to the customer in the form of higher purchase prices for the machines, because inventory control is required to place the proper roller on the proper machine. Further, the customer would be locked into a particular film formulation, and would be required to buy new rollers if the customer elected to use a less expensive or more versatile film formulation in the future. It should also be noted that tackiness additive built up on the portion of the roller which was in contact with film web.
It has also been determined that all types of prior art rollers permit slippage of film web across the circumference of the rollers during the pre-stretch stage of film web application to a load. The pattern of slippage varies both along the length of the rollers and from one revolution of the rollers to the next. This slippage reduces the overall stretch of film web and further contributes to economic waste. Because the slippage varies during wrapping, operators cannot rely on their own adjustments to prevent film web rupture at high throughput, and so must reduce the stretch ratio and the web output speed to build in a safety margin.
Many of these problems arise due to the non-uniform nature of the film web. Although any commercial roll of web appears undifferentiated to the eye, there are actually significant variations in thickness and material phase across any given cross-section in contact with a roller. Thicker regions cause adjacent thinner regions to stand off the roller, thus reducing the ability of the roller to isolate forces on an upstream web portion from forces on a downstream web portion. The smooth, continuous surfaces of the prior art rollers distribute compression from thick web regions too broadly, so the roller surface deflects minimally and the thick web regions never sink in enough to allow contact of adjacent thin regions with the roller surface. When thin regions stand off, web is stretched by forces both upstream and downstream of the roller. There are also significant variations in the material phase across any given cross-section in contact with a roller: some local regions are more crystalline and brittle while other adjacent regions are more amorphous and pliable. Amorphous regions elongate more rapidly than adjacent crystalline regions. Under extremely high forces, such as when thin web sections stand off from the roller, the transition interface between a rapidly-stretching amorphous region and a resistant crystalline region is very often the site of film web rupture.
It can be appreciated that the combination of surface wear, tackiness additive buildup, circumferential slippage, and film web adherence to the surface has significantly contributed to the frequency of film web failure and resultant down time, and it has also stimulated the operator response of reducing the wrapping system operating speed and film web stretch ratio in order to minimize failures. It is therefore clear that there exists a need in the prior art to improve the structure of pre-stretch rollers so that system performance is more reliable and consistent, permitting higher speeds and higher levels of pre-stretch on the web.